Chapter 14 Notes

Chapter 14 Notes - Chapter 14 Chemical Equilibrium Many...

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Unformatted text preview: Chapter 14 Chemical Equilibrium Many reactions fail to go to completion. [Reactants] stop decreasing, and [Products] stop increasing. The reaction reaches equilibrium . If there are more: products than reactants = product favored. reactants than products = reactant favored. Characteristics of Chemical Equilibrium Equilibrium occurs when [A], [B], [C] and [D] stop changing - a dynamic equilibrium: Species do not stop forming OR being destroyed. Rate of formation = rate of removal. Concentrations are constant. Reactants convert to products a A + b B c C + d D BUT , products can also react to make reactants. Equilibrium is Dynamic N 2 (g) + 3 H 2 (g) 2 NH 3 (g) or with: 2 mol NH 3 Equilibrium is Independent of Direction Start with: 1 mol N 2 + 3 mol H 2 They will form identical equilibrium mixtures Equilibrium & Catalysts Catalysts do not affect equilibrium concentrations. Catalysts: speed up the forward reaction but also speed up the reverse reaction. decrease the time needed to reach equilibrium. 2-butene isomerization: At equilibrium: rate forward = rate reverse The Equilibrium Constant or = [ trans ] [ cis ] k forward k reverse This ratio is named the equilibrium constant, K c : The isomerization is elementary, so: k forward [ cis ] = k reverse [ trans ] c concentration based K c = = = 1.65 (at 500 K) k forward k reverse [ trans ] [ cis ] The Equilibrium Constant T (K) 500 600 700 K c 1.65 1.47 1.36 Equilibrium constants are temperature dependent: The Equilibrium Constant For a general reaction: a A + b B c C + d D K c = = [C] c [D] d [A] a [B] b k forward k reverse Products raised to stoichiometric powers divided by reactants raised to their stoichiometric powers The Equilibrium Constant Examples N 2 (g) + O 2 (g) 2 NO(g) K c = [NO] 2 [N 2 ] [O 2 ] S 8 (s) + O 2 (g) SO 2 (g) 1 8 K c = [SO 2 ] [O 2 ] S 8 is ignored. All pure solids are omitted from K c . [Solid] is constant throughout a reaction. pure solid concentration = [S 8 ] = d S 8 / mol. wt S 8 d and mol. wt. are constants, so [S 8 ] is constant. This constant factor is absorbed into K c . Equilibria Involving Pure Liquids & Solids Pure liquids are omitted for the same reason. density mol. wt g / L g / mol Equilibria in Dilute Solutions Water is omitted from K c in dilute solution reactions: NH 3 (aq) + H 2 O() NH 4 + (aq) + OH- (aq) It is in large excess (pure water = 55.5 M). Even if consumed or produced, [H 2 O] constant. The constant factor is incorporated into K c . K c = [NH 4 + ][OH- ] [NH 3 ] = 1.8 x 10-5 (at 25 C) (Units are customarily omitted from K c ) The Equilibrium Constant What is the equilibrium constant for: SiH 4 (g) + 2 O 2 (g) SiO 2 (s) + 2 H 2 O(g) ?...
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This note was uploaded on 01/27/2012 for the course CH 1213 taught by Professor Xia during the Spring '07 term at Mississippi State.

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Chapter 14 Notes - Chapter 14 Chemical Equilibrium Many...

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